It is previously known to locate two paper coaters at a coating station around a single back up or backing roll to coat the same side of a moving paper web. For example, in U.S. Pat. Nos. 4,250,211, 4,310,573, or 4,512,279, a dip roll coater unit and a short dwell time applicator (SDTA) or coater unit are mounted about a single backing roll. Additionally, it has been known to locate two coater units on a backing roll with one coater coating one side of the web and the other coater coating the opposite side of the web. Heretofore, it has been unknown to locate three coaters coating the same side of the web about a single backing roll. The physical size of the coaters, the need to place them at certain locations on the backing roll and technical coating requirements have dictated that two or fewer coaters be placed at a coating station about a single backing roll. However, as individual coaters became more complex, such as the coater with an integral and a separate doctor or blade shown in U.S. patent application Ser. No. 375,241, filed Jul. 3, 1989, now abandoned and wherein it is desirable to use such a more complex coater in pairs or with other types of coaters, it becomes very difficult to accommodate even two of them around a single backing roll coating station, while yet permitting efficient operation and proper maintenance of the individual coaters.
While multiple coaters could be accommodated on multiple backing roll coating stations, such an approach is cost prohibitive and from a technical standpoint also prohibited. Many times successive coatings on the same side of the web must be applied by the multiple coaters in a time frame which would not permit the multiple backing roll coating stations approach. For that matter, it is not possible for the coaters to be merely evenly spaced about the web wrapped portion of the circumference of the backing roll, as most coaters for operational reasons can only be used or located in specific positions on the backing roll. For example, generally trailing blade coaters cannot be located in just any quadrant of the backing roll. An inverted blade coater is usually located on one of the lower quadrants of the backing roll. A dip roll coater is usually located near the bottom of the backing roll. Additionally, a paper mill requires a great deal of flexibility in which type coaters will be used, making highly desirable the location of three coater or blade units for coating or blading the same side of the web at a coating station around but a single backing roll.
Another disadvantage in using multiple coaters on a single backing roll is it becomes difficult not only to maintain the coaters, but in fact to even operate them. For example, with modern paper machines and coaters now approaching 33 feet (10.0 meters) in width, referred to as the cross machine direction, it becomes extremely difficult to withdraw coating doctored or bladed off by the coater blade over such a long cross machine distance. As the coatings used can be many times more viscous than water, it would be difficult to remove doctored or bladed off coating using a wide but relatively shallow depth, gravity flow, coating overflow pan. Heretofore, in order to insure the bladed off coating did not spill over a gravity flow, coating overflow pan, it was necessary for the overflow pan to be relatively deep, say eighteen inches (20 cm)or more in depth. The requirement for such a deep pan, itself, can make it difficult, if not impossible, to put two or more coater or blade units around a single backing roll but yet permit the coaters or blade units to be properly adjusted.
Attempts in the past have been made to use an auger to withdraw coating from a coater overflow pan. See U.S. Pat. No. 3,518,964. In that patent the auger was very short, extended in a machine direction, and not a cross machine direction, and operated in a very deep pan. Such pan and auger installation was totally incapable of being used in a multiple coater arrangement, as there is insufficient room, and that installation would not function to remove coating from a pan approximately 33 feet (10.0 meters) wide in an cross machine direction.
Further, in order to build such a wide machine and coater unit, it is also necessary to control and prevent deflection of the coater unit and particularly of any blade or blade unit which must be extremely accurately placed across the entire width of the web, while the coater or blade unit is usually only supported at its widely spaced apart sides or ends.
Because of the great width of coaters, approximately 33 feet (10.0 meters) and getting wider, and the requirement that the coater and doctor or blade positions be accurately controlled, to perhaps as accurate as a few ten thousandths of an inch (several thousandths of a millimeter), it is very difficult to place several coaters and/or blade units on but a single backing roll. Such accuracy requires very rigid and consequently large cross beams to carry the weight of the units, which can be 8 tons or perhaps more. However, the larger the cross beam, the greater the deflection problem becomes due to the increased weight of the cross beam itself. Even a large cross beam would have considerable deflection due solely to the coater or blade unit it carries and the beam's own weight. One approach used to limit the deflection is by pre-stressing the cross beam, itself, to counter the anticipated deflection due to gravity and operational loading. Such a deflection control system is shown in U.S. Pat. Nos. 4,907,528 or 5,005,515.